1. Field of the Invention
The present invention relates to a hydraulic control valve, and more particularly to, a hydraulic control valve having a holding valve with improved response characteristics in which an actuating path for the holding valve is provided to improve response characteristics of the holding valve, thereby reducing natural drop of a hydraulic cylinder.
2. Description of the Related Art
Generally, a heavy construction equipment such as an excavator is provided with working devices such as a boom, an arm, and a bucket. The heavy construction equipment is driven by a hydraulic cylinder and performs various operations. The hydraulic cylinder is operated by hydraulic oil supplied from a hydraulic pump. A hydraulic control valve is provided between the hydraulic pump and the hydraulic cylinder to supply the hydraulic oil from the hydraulic pump to the hydraulic cylinder. The working devices are driven in such a manner that the hydraulic control valve is operated to control pressure, direction, and flow of the hydraulic oil supplied to the hydraulic cylinder as a driver manipulates a lever.
The working devices are massively manufactured to endure great load and tough working conditions. In a neutral state that the working devices are idle, some of the hydraulic oil may be leaked through a spool of the hydraulic control valve and a gap of a valve housing due to load of the massive working devices and load of freight. For this reason, the working devices may naturally be dropped. Therefore, a hydraulic system fixed to the heavy construction equipment includes a holding valve that prevents natural drop due to load in a neutral state from occurring.
FIG. 1 is a circuit diagram illustrating a related art hydraulic control valve having a holding valve, and FIG. 2 is a partially sectional view illustrating a related art hydraulic control valve having a holding valve.
A related art hydraulic control valve 100 includes a valve housing 110, a control spool 120 provided in the valve housing 110 and switched by pilot signal pressures ‘a’ and ‘b’ to control flow and direction of a hydraulic oil, and a holding valve 130. The valve housing 110 includes a pump path 111 communicating with a hydraulic pump 2, a tank path 112 communicating with an oil tank 4, and first and second cylinder ports 113 and 114 communicating with a hydraulic cylinder 3.
As shown, if the control spool 120 is in a neutral state, the hydraulic oil of the hydraulic pump 2 is ejected to the oil tank 4. The holding valve 130 includes a holding poppet 131 provided between the second cylinder port 114 and the control spool 120, and a drain valve 132 supplying or cutting the hydraulic oil to or off a rear pressure chamber 134 of the holding poppet 131. In a state that the control spool 120 is neutral, the hydraulic oil of the hydraulic cylinder 3 flows into the rear pressure chamber 134 of the holding poppet 131 through the cylinder port 114 and an orifice 133 of the holding poppet 131. Therefore, the holding poppet 131 is pressurized up and down when viewed from the drawing so that the hydraulic oil of the hydraulic cylinder 3 may not be leaked into the oil tank 4.
In this state, if the pilot signal pressures ‘a’ and ‘b’ act on pilot ports 120a and 120b at left and right sides of the control spool 120, the control spool 120 is switched to the left or right side so that the hydraulic oil of the hydraulic pump 2 may flow into the first cylinder port 113 or the second cylinder port 114 to drive the hydraulic cylinder 3.
If the control spool 120 is switched to the left side as the pilot signal pressure ‘a’ acts on the right pilot port 120a of the control spool 120, the hydraulic oil of the hydraulic pump 2 flows into the first cylinder port 113 to drive the hydraulic cylinder 3. At the same time, the pilot signal pressure ‘c’ acts on the drain valve 132 so that the drain valve 132 may be switched to the right side and the holding poppet 131 may be opened to eject the hydraulic oil of the hydraulic cylinder 3 to the oil tank 4.
However, in this case, if the control spool 120 is switched to the neutral state, a connecting line 115 connected to the holding poppet 131 is cut off. Thus, the connecting line 115 and the second cylinder port 114 are subject to the same pressure. At this time, since the hydraulic oil supplied from the hydraulic cylinder 3 through the opened holding poppet 131 acts on the control spool 120 through the second cylinder port 114 and the connecting line 115, the hydraulic oil may be leaked out through the control spool 120 and the valve housing 110 due to load of the hydraulic cylinder 3. Since the leakage oil is not small quantity but great quantity, it acts to cause natural drop of the working devices.
If the pressure of the connecting line 115 becomes smaller than that of the second cylinder port 114 as the hydraulic oil of the connecting line 115 leaks while the working devices are naturally dropped, the holding poppet 131 is closed to prevent the leakage oil from occurring. However, time delay occurs until the holding poppet 131 is closed after the pressure of the connecting line 115 becomes smaller than that of the second cylinder port 114. Leakage oil occurs for the time delay.
As described above, since response speed of the holding valve 130 is slow in the related art hydraulic control valve 100, the hydraulic oil supplied to the rear pressure chamber 134 has been conventionally increased by increasing a sectional area of the orifice 133 of the holding poppet 131 to avoid excessive natural drop of the hydraulic cylinder 3. However, this failed to fundamentally solve time delay of the holding valve 130. Also, if the sectional area of the orifice 133 is increased, hunting occurs when the working devices are initially operated or the rear pressure increases due to increase of the hydraulic oil in the rear pressure chamber 134, thereby causing unstable working pressure.